The immunological function of intestinal epithelial and mononuclear cells has received intense investigative attention in recent studies of innate and adaptive immunity. In contrast, the role of the extracellular matrix, or stroma, in local immune responses has not been critically evaluated and is poorly understood. In view of the newly appreciated importance of the stroma in immunological cross-talk and regulation in other organs [1-6] and the role of the intestinal stroma in regulating mucosal macrophage differentiation [7-9], we propose to investigate the function of human intestinal stroma in the regulation of tolerogenic and pro-inflammatory T cell function. Importantly, mechanisms that regulate T cells appear to vary among different mucosal compartments, since T cells isolated from the normal gastric mucosa proliferate more strongly than T cells from the intestine, and gastric TGF levels are significantly lower than intestinal levels, as we show here. Therefore, we hypothesize that (1) Discordant CD4+ T cell proliferation in gastric and intestinal mucosa is due to different levels of TGF in these mucosal compartments in turn due to different bacterial loads in the stomach versus the intestine, and that (2) Stroma-associated factors from normal mucosa, particularly TGF, promote tolerogenic T cells in normal mucosa, whereas other mucosa-derived factors from inflamed mucosa, particularly IL-6 and IL-1, promote pro-inflammatory and regulatory T cell subsets. We will test these hypotheses with the following Specific Aims: " Specific Aim 1. Determine whether the discordant CD4+ T cell proliferation in normal gastric versus intestinal mucosa is due to differences in levels of stroma- associated TGF. " Specific Aim 2. Determine whether the reduced level of TGF in normal gastric mucosa is due to the sterile nature of the normal gastric mucosa. " Specific Aim 3. Determine whether the mucosal TGF/IL-6 axis promotes tolerogenic CD4+ T cells in normal gastric and intestinal mucosa via TGFhi/IL-6lo, and proliferation of pro-inflammatory and regulatory CD4+ T cells in inflamed gastric and intestinal mucosa via TGF hi/IL-6hi. The gastrointestinal mucosa is the largest mucosal surface to interact with the external environment, maintaining in healthy tissue a homeostatic balance between tolerogenic responses to commensal bacteria and food antigens and necessary protective immunity against pathogens that invade the lamina propria. Little is know about the immunoregulatory mechanisms underlying the regulation of homeostasis in different mucosal compartments (stomach and small intestine) and how this control is lost in T cell-mediated disease and inflammation. This application will elucidate the role played by the mucosal microenvironment and in particular, the balance between tolerogenic and pro-inflammatory cytokines that control the proliferation of effector T cells in the gastric and intestinal mucosae.